This invention relates to a glass powder which can be crystallized to yield a sintered glass ceramic containing hexagonal cordierite as the principal crystalline phase.
As a consequence of the good mechanical and electrical properties of cordierite, it is highly suitable as a substrate for electronic components, in particular for multilayer circuit boards. Since pure cordierite has very high sintering temperatures at which conductor circuitry applied before sintering would be destroyed, the starting point is a glass powder which can be sintered at relatively low temperatures and simultaneously substantially converted into a crystalline phase.
It is known that glass powders whose composition corresponds to stoichiometrically pure cordierite have only a limited sintering capability. Pressing and firing at temperatures of up to 1200.degree. C. yield sintered glass ceramics which are porous and have low mechanical strengths. The poor sinterability is due to premature surface crystallization of the glass particles, which causes a dramatic increase in the viscosity of several orders of magnitude, which prevents further sintering of the glass phase. A further disadvantage is that MgO-containing high-quartz mixed crystal phases initially form. Representatives from this series of mixed crystals are .mu.-cordierite having an MgO:Al.sub.2 O.sub.3 :SiO.sub.2 oxide ratio of 2:2:5 and a magnesium aluminosilicate (MAS) having a ratio of 1:1:4. The existence of these metastable crystal phases additionally delays the formation of hexagonal cordierite.
Glass powders having a modified composition and improved sinterability have therefore already been disclosed. Thus, U.S. Pat. No. 3,926,648 states that the sinterability is improved by addition of 0.5-2% by weight of K.sub.2 O and/or Cs.sub.2 O. However, the sintered elements produced from these glass powders have impaired electrical and dielectric properties and are therefore generally unsuitable for use for electrotechnical and electronic components, in particular if the requirements are stringent.
German Patent 26 02 429 provides a modification by adding 0.7-5.6 mol % of one or more of the modifying oxides BaO, PbO, SrO and CaO. These oxides are capable of forming mixed crystals in the cordierite structure. However, the low residual glass phase content limits the sinterability of the powder, so that the powders are principally used as coatings on ceramic, glass or glass-ceramic articles.
German Patent 29 01 172 describes two different sintered glass ceramics, firstly a glass ceramic based on .beta.-spodumene as the most important crystalline phase, and secondly, a sintered glass ceramic based on cordierite as the most important crystalline phase. The structure of the latter sintered glass ceramic additionally contains clinoenstatite and in some cases .mu.-cordierite in addition to hexagonal cordierite. This complex structure makes it necessary to observe narrow tolerances for all the process parameters, such as chemical composition, powder properties and sintering schedule, in order to achieve the formation of a uniform structure, and thus reproducible properties. These sintered glass ceramics, therefore, are produced by a complex process on the one hand and are difficult to match to the various processing conditions of various users on the other hand.
U.S. Pat. No. 4,540,671 describes a glass powder which can be sintered to yield a sintered glass ceramic containing cordierite and a high-quartz mixed crystal phase in solid solution. However, the occurrence of the metastable high-quartz mixed crystal phase results in the disadvantages mentioned above.